Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
  • H01S3/2383—Parallel arrangements
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
  • H01S3/1123—Q-switching
  • H01S3/117—Q-switching using intracavity acousto-optic devices
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/16—Solid materials
  • H01S3/1601—Solid materials characterised by an active (lasing) ion
  • H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
  • H01S3/1611—Solid materials characterised by an active (lasing) ion rare earth neodymium
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/07—Construction or shape of active medium consisting of a plurality of parts, e.g. segments
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
  • H01S3/08072—Thermal lensing or thermally induced birefringence; Compensation thereof
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
  • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
  • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
  • H01S3/1123—Q-switching
  • H01S3/127—Plural Q-switches

Definitions

• the present invention relates to a laser cavity (in English "optical resonator") of high peak power and medium power and high rate of recurrence, while being minimized in cost and complexity. It also relates to the association of several of these cavities, in particular for exciting a light generator in the extreme ultraviolet. The invention thus applies more particularly to the generation of light in the extreme ultraviolet range.
• the radiation belonging to this domain also called “EUV radiation” has wavelengths which range from 8 nanometers to 25 nanometers.
• the EUV radiation which can be obtained by making light pulses interact, generated with the device object of the invention, and an appropriate target have many applications, in particular in materials science, in microscopy and very particularly in microlithography, for manufacture integrated circuits with a very high degree of integration. For this latter application, it is particularly advantageous to have a rate of high recurrence, which is very difficult to obtain for high peak power lasers.
• the invention applies to any field which requires an excitation laser of the same kind as those which are needed in microlithography.
• EUV lithography is necessary in microelectronics for the production of integrated circuits whose dimensions are less than 0.1 micrometer.
• EUY radiation sources several of these sources use a plasma generated by a laser.
• the peak power of the laser light must be very high (of the order of 10 11
• a pulse laser is used for this purpose, delivering for example an energy of the order of 300 mJ per pulse or more.
• the invention uses, for example, YAG lasers doped with neodymium, lasers which have known many developments in many industrial fields.
• YAG lasers doped with neodymium lasers which have known many developments in many industrial fields.
• other solid-state lasers that is to say whose amplifying medium is solid, can be used in the present invention.
• pumping by laser diodes it is known to use pumping by laser diodes.
• M 2 The theoretical lower limit of M 2 is equal to 1 but the higher the power of the laser, the more the value of M 2 increases. It commonly reaches several tens with a YAG laser doped with neodymium, also called an Nd: YAG laser.
• This article describes a laser installation using three identical modules placed in parallel, each of these modules consisting of the laser produced by the company TRW and described in the following document.
• light pulses are generated in a basic laser comprising a very small low energy oscillator ( less than 10 mJ per pulse) and of low average power (less than 15W), and they are amplified by numerous passages in amplifier stages with bars or plates.
• the first stage or floors are generally traversed twice (round trip, hence the name of double pass amplifier), which requires working with a polarized beam and d '' use a polarizer (for example a polarizing cube) so that the return path does not return to the oscillator but is directed to another optical path where the amplification will be continued.
• a polarizer for example a polarizing cube
• Nd: YAG laser comprising, in a laser cavity, two Nd: YAG bars, a polarization rotator between these bars, two acousto-optical modulators respectively on either side of the two bars and a diverging lens between each modulator and the corresponding bar.
• the average output power of the laser cavity is 260 W, with a recurrence rate of 10 kHz.
• the embodiment described in this document does not take into account a significant problem linked to the devices for triggering light pulses, in particular acousto-optical devices used in the laser described in this document: their operation depends on the divergence of the laser beam.
• the acousto-optical triggers essentially comprise an acousto-optical crystal and a control device, and operate as follows.
• the control device When it receives an electrical signal, the control device emits a radio frequency excitation wave in the crystal, which generates a Bragg grating in this crystal. In the absence of excitation, this crystal lets pass the incident rays, which, under the nominal operating conditions do not arrive according to normal to the entry face of the crystal, but by making with it a Bragg angle.
• the radio frequency wave When the command is activated, the radio frequency wave generates the Bragg grating, which then deflects the incident light rays; the deflection angle is sufficient for these rays to exit the laser cavity, which corresponds for the laser to a beam cut.
• this limiting angle is practically the same as the value of the angle which exists between the directions of the beams diffracted at first and second order by the Bragg grating formed in this crystal when it is excited (typically about 4 mrad ).
• the rays whose angle of incidence is close to this angle are not correctly intercepted when the crystal is excited.
• the rays whose incidence exceeds this angle are also not deflected properly, but moreover they return towards the central part of the laser cavity since their incidence is included in the angular acceptance of this cavity.
• the present invention aims to solve both the problems inherent in the MOPA structure implemented in the embodiments described by documents [5] to [7], and the problems inherent in structures having an oscillator delivering high power but whose stability is affected by the limitations of acousto-optical triggers, as in the embodiment described in document [8].
• the invention aims to solve them, by means of a laser cavity capable of having a high peak power and a high rate of recurrence, and by means of the association of this cavity with other identical cavities to constitute a device. laser which achieves higher performance with respect to peak power, than the devices disclosed in documents [5] to [8], while being less complex, less expensive and more reliable to operate.
• the laser devices disclosed by document [5] aim to obtain short pulse durations, from 5ns to 20ns, which the person skilled in the art considers favorable for obtaining a plasma. very emissive.
• the subject of the present invention is a laser cavity (in English: "optical resonator") with solid amplifying medium, this laser cavity being pulsed and pumped by diodes operating continuously, characterized in that it comprises: at least two laser bars, at least one means for triggering light pulses, this trigger means being located in the part of the cavity where the laser beam generated by the cavity diverges the least, and
• the part of the cavity where the beam diverges the least is the part located between the two bars.
• the parts of the cavity located outside the bars, between one of the bars and one of the mirrors of the cavity are the parts where the beam diverges the most.
• the laser bars are of isotropic material such as Nd: YAG or Yb: YAG
• Nd: YAG or Yb: YAG it is necessary, in order to obtain the beam quality specified for the microlithography industry, to add into the cavity, along the beam path , a means of rotation of the polarization, in each of the spaces formed by two successive bars, this rotation preferably being 90 °.
• the slight convergence produced by certain laser bars is corrected, in particular in Nd: YAG, by placing on the beam, in the middle of each interval between two adjacent bars, a lens having an opposite effect on convergence.
• the laser material from which the laser bars are made is chosen from the group comprising Nd: YAG, Nd: YLF, Nd: YALO, Yb: YAG, Nd: Sc0 3 and Yb: Y 2 0 3 .
• the cavity which is the subject of the invention comprises two bars made of laser material, preferably substantially identical, polarization rotation means placed in the cavity, between these two bars, and two means for triggering the pulses, placed between the two. bars, on either side of the polarization rotation means.
• the triggering means are of acousto-optical type.
• the laser cavity according to the invention can, according to an alternative embodiment, be associated with one or more single-pass laser amplifiers, pumped by diodes, the rod of each amplifier being stressed over its entire length at the saturation fluence of the material of the bar or above this fluence.
• this fluence reaches at least three times the saturation fluence of the material.
• the laser cavity is characterized by its ability to deliver, in a stable manner, a high fluence, without it being necessary to make the beam it generates converge. It can maintain the parallelism of this beam and reach or exceed this saturation fluence over the entire length of the bar. In the preferred application which will be detailed later, this fluence is even worth ten times the fluence of saturation of the material.
• the invention also relates to the association of at least three cavities of the above type, arranged in parallel but whose beams they generate are directed towards the same target.
• the laser device resulting from this association of these cavities is characterized in that it comprises: - at least three pulse laser cavities (in English: "optical resonators"), with solid amplifying medium, these cavities being in accordance with the object laser cavity of the invention, and optical means for sending these light pulses substantially at the same location of a target and substantially at the same time at this location, and in that the device also comprises means for controlling the pulsed laser cavities , these control means being provided so that all the pulses reach the target almost at the desired time with an accuracy better than 5 ns, and preferably better than 1 ns.
• the laser cavities are associated with one or more single pass amplifiers.
• the means for triggering each pulse laser cavity comprise two triggers placed in this cavity, on either side of the polarization rotation means, between the latter and the bars made of laser material.
• the means for sending the light pulses comprise means for sending these light pulses to the target along the same path.
• this device also comprises means for modifying the spatial distribution of the light pulse resulting from the addition of the light pulses supplied by the laser cavities.
• the means for controlling the laser cavities are further able to modify the temporal distribution of the light pulse resulting from the addition of the light pulses supplied by the laser cavities, in order to create composite pulses.
• the profile of each composite pulse comprises a first plasma ignition pulse intended to be created by interaction of light pulses with the target, a time interval where the light energy emitted by the laser is minimal during growth plasma and then a second pulse, composed of several elementary pulses, according to a sequence depending on the growth of the plasma.
• the device which is the subject of the invention is preferably able to send a first beam very focused on the target, then to apply the rest of the light energy to the target with a focusing wider.
• the target to which the light pulses emitted by the laser cavities of the device object of the invention are sent may be provided to provide light in the extreme ultraviolet range by interaction with these light pulses.
• the present invention is not limited to obtaining EUV radiation. It applies to any field where there is a need for high peak power laser beams, directed at a target.
• a spatial superimposition is used and, in a particular embodiment, a temporal sequencing.
• spatial superposition is meant the superposition of a plurality of laser beams at substantially the same location on the target, substantially at the same time.
• Substantially at the same time means that the time offsets between the various elementary pulses respectively supplied by the laser cavities of the laser device are small compared to the recurrence period of these laser cavities. This superposition multiplies the energy per pulse and the peak powers.
• a first light beam FI and a second light beam F2 are seen in section in FIG. 1, in a plane which is defined by two perpendicular axes Ox and Oy, the axis common to the two beams being the axis Oy.
• the two beams have substantially a symmetry of revolution around this axis Oy and are focused in the vicinity of point 0, substantially in the observation plane which is defined by the axis Oy and by an axis perpendicular to the axes Ox and Oy and which passes through point 0.
• the focalizations of the two beams are different, the first beam FI being more focused than the second F2.
• FIG. 2 shows the variations of the illumination I in the observation plane as a function of the abscissa x counted on the axis Ox.
• the illumination produced by this beam FI on the axis Oy is multiplied by twenty five compared to that produced by this beam FI when the two beams have the same power.
• bursts of pulses in which the time offsets between two pulses of two elementary laser cavities are very small compared to the time of recurrence between two bursts. Such bursts can be considered as composite pulses.
• the invention preferably uses this sequencing, over time, of various laser pulses. It allows, for example, the following sequencing.
• a first pulse very focused on the target ignites a plasma, then, during the time when the plasma grows, the target is subjected to a minimal or no illumination, and when the plasma reaches the diameter of the beam F2, the target is subjected to a maximum of light power. It is then advantageous to devote to the first pulse an energy lower than that devoted to the remainder of the composite pulse according to FIG. 3.
• the amplitudes A of the light pulses are represented as a function of time t.
• the latter includes a pre-impulse ("prepulse")
• the lifetime of the upper level of the laser cavity which is close to 250 microseconds, requires working at a rate greater than 5 kHz to properly extract the deposited light power.
• the present invention unlike the prior art, makes it possible to obtain high peak powers by associating an unfavorable point with this peak power (point c), and a favorable point (point a) whose weight is all the more important. that we increase the number of elementary laser cavities.
• Point (b) is only a possibility of adapting the invention as well as possible to its applications.
• points (a), (b) and (c) can be used simultaneously, and this combination of points favorable and unfavorable to obtaining high peak powers goes against the prior art.
• a laser device according to the invention can be much simpler than those of the prior art because this device can operate without using the series connection of amplifiers.
• the increase in the number of laser cavities also allows a device according to the invention to be less sensitive to an incident relating to the instantaneous performance of one of the laser cavities.
• FIG. 1 and 2 schematically illustrate the use of two focused laser beams differently to obtain a large local illumination and have already been described
• FIG. 3 schematically illustrates an example of composite light pulse usable in the present invention and has already been described
• FIG. 4 is a schematic view of the association of several cavities laser according to the invention with a view to creating a device for exciting a light source in the extreme ultraviolet
• FIG. 5 schematically illustrates a particular embodiment of the laser cavity which is the subject of the invention
• FIG. 6 and 7 schematically and partially illustrate other examples of the invention, allowing spatial multiplexing of the elementary laser beams respectively generated by several laser cavities.
• a laser cavity according to the invention is shown in Figure 5 to which we will return later. It can be followed by one or more single pass amplifiers.
• FIG. 4 The association of several pulse laser cavities according to the invention in order to create a device for exciting a light source in the extreme ultraviolet is schematically represented in FIG. 4.
• the device of FIG. 4 comprises more than three pulse laser cavities, which are also called pulse lasers, for example ten, but only three of them are represented in this figure 4 and have the references 2, 4 and 6 respectively.
• the light beams 8, 10 and 12 (more precisely the light pulses), which are respectively supplied by these pulse laser cavities 2, 4 and 6, are sent, via a set of mirrors 14, substantially at the same point P of a target 16 and substantially at the same time at this point P.
• the means 18 for controlling the lasers have also been shown, making it possible to obtain the light pulses.
• FIG. 4 also shows focusing means 20, 22 and 24 which are, for example, achromatic doublets, provided for focusing the light beams 8, 10 and 12 respectively on the point P of the target 16.
• the lasers and the target are chosen to provide, by interaction of the light beams with this target, EUV radiation 26.
• the target comprises for example a jet 28 of aggregates (for example of xenon ) which come from a nozzle 30.
• This EUV radiation is used for example
• the block 34 of FIG. 4 symbolizes the various optical means serving to shape the EUV radiation before it reaches the integrated circuit 32.
• the lasers 2, 4 and 6 are identical or almost identical and capable of providing light pulses.
• Each of them comprises two pumping structures 36a and 36b whose aberration and birefringence are low.
• the structure 36a (respectively 36b) comprises a laser bar 38a (respectively 38b) which is pumped by a set of laser diodes 40a (respectively 40b) operating continuously.
• the material chosen for our experiments is Nd: YAG, whose saturation fluence is 200 mJ / cm 2 ;
• prepulse pre-impulse
• Each laser cavity directly produces a power of 300W at 10kHz, with a beam quality compatible with multiplexing, the pulse duration being 50ns and its energy of 300mJ.
• the fluence of the beam at the exit of the cavity is 2.3 J / cm 2 , that is to say nearly 10 times the fluence of saturation of the Nd: YAG material.
• the focusing of the beam produced by each of the lasers 2, 4 and 6 over an area of 50 ⁇ m in diameter of the target then leads to a peak power of 3 ⁇ 10 10 W / cm 2 to 6 ⁇ 10 10 W / cm 2 .
• no light amplifier is used with lasers 2, 4 and 6.
• these amplifiers would operate with a relatively low gain, but with an optimal extraction of the energy deposited in the bar of this amplifier taking into account the fluence almost 10 times greater than the fluence of saturation of the material of this bar.
• Figure 5 is a schematic view of a pulse laser cavity according to the invention. It is constituted like any one of the cavities 2, 4, 6 and thus comprises the structures 36a and 36b as well as the mirrors 42 and 44, the polarization rotator 46 and / or the lens 46a and the means for triggering pulses 50 and 52 which will be discussed later.
• a light amplifier 36c is placed at the output of this laser cavity.
• This amplifier 36c comprises a single-passage laser bar 38c, which is pumped by a set of laser diodes 40c operating continuously.
• the control means 18 are then provided for controlling this amplifier 36c.
• the latter is substantially identical to structures 36a and 36b and its laser bar 38c is preferably made of the same laser material as the laser bars 38a and 38b.
• This laser material is chosen from Nd: YAG (preferred material), Nd: YLF, Nd: YALO, Yb: YAG, Nd: Sc0 3 and Yb: Y 2 0 3 .
• Each laser cavity is delimited by a first highly reflective mirror 42 (reflection coefficient R equal to 100% for example 1064nm) and by a second mirror 44 which is partially reflective (R of the order of 70 % to 80%) to pass the light beam generated by this laser cavity.
• mirrors are preferably curved and their radii of curvature are calculated so as to allow the beam to have a small divergence, such that the parameter M 2 is approximately equal to 10.
• the length of the cavity is chosen as a function of the duration of the pulses.
• the two curved mirrors can be replaced by two sets, each comprising a divergent lens and a plane mirror.
• a polarization rotator 46 of 90 °, at any location between the two laser bars 38a and 38b.
• a slightly divergent lens 46a can be used, exactly halfway between the two bars.
• the diameter of these laser bars is between 3 mm and 6 mm.
• each bar of Nd: YAG is pumped by 40 laser diodes, each of these diodes having a power of 30W and emitting at 808nm.
• the pumping of each bar is homogeneous.
• each laser pulsed there are in the cavity on the path of the beam, at the place where it diverges the least, that is to say between each of the bars and the polarization rotator, means for triggering the acoustic pulses. optical to allow triggering at high speed of these pulses.
• Each of these acousto-optical triggers uses a silica crystal, works in compression mode with a radio frequency power of 90 W at 27 MHz, this power being applied to the crystal by a 4 mm transducer.
• two acousto-optical deflectors 50 and 52 of the defined type are used above, which are controlled by the control means 18 and arranged in the space delimited by the laser bars 38a and 38b, on either side of the polarization rotator 46. These two acousto-optical deflectors 50 are used and 52 to block the cavity with gains corresponding to the average power mentioned above.
• the control means 18 trigger the operation of the EUV source and make it possible to adapt its characteristics to the needs of microlithography. If necessary, they determine the simultaneity of the light pulses of the lasers 2, 4 and 6 at the target. If the optical paths are of significantly different lengths, they make it possible, in particular, to compensate for these differences and to manage the trips of all the acousto-optical deflectors contained in the device of FIG. 4 so that synchronism is achieved for light pulses.
• the control means 18 comprise: means (not shown) for generating the supply currents of the pumping diode lasers 40a and 40b (and possibly 40c) and means (not shown) for generating radio frequency currents modulated, intended to control each " pair of acousto-optical deflectors 50 and 52 in an almost synchronous manner, the offset of these deflectors preferably being less than 1 ns.
• control means 18 are provided for controlling the lasers 2, 4 and 6 as a function of signals for measuring the radiation of the plasma (generated by interaction of the laser beams with the target 16), supplied by one or more suitable sensors such that the sensor 54, for example one or more fast silicon photodiodes with spectral filtering; for EUV radiation, this filtering can be carried out by zirconium, and by a multilayer Molybdenum-Silicon mirror, possibly doubled; in the case where the growth rate of the plasma is observed, it is necessary either to modify this filtering, or to add one or more other fast photodiodes whose filtering is closer to the visible spectrum.
• the control means 18 are also provided for controlling the lasers 2, 4 and 6 as a function of signals for measuring the energy of the light pulses of the lasers 2, 4 and 6, signals which are respectively supplied by appropriate sensors 56, 58 and 60, for example fast silicon photodiodes with integrating means, and
• the optical means formed by the deflection mirrors 14 and the achromatic focusing doublets 20, 22 and 24 are chosen to allow a spatial superposition with position fluctuations which are less than a small percentage, for example of the order of 1% to 10%, of the diameter of the focal spot (point P).
• the laser device of FIG. 4 further comprises means provided for modifying the spatial distribution of the pulse resulting from the addition of the light pulses respectively emitted by the lasers 2, 4 and 6. These means, symbolized by the arrows 74, 76 and 78, are for example provided for displacing the achromatic doublets 20, 22 and 24, so as to modify the sizes of the focal spots respectively provided by these doublets.
• the control means 18 can be provided for temporally shifting, with respect to each other, the light pulses emitted by the lasers 2, 4 and 6, by appropriately shifting the triggers of the lasers with respect to each other. It should be noted that the laser device of FIG. 4 is not polarized, unlike other known laser devices, for example those described in document [5].
• N 10 for example
• FIG. 6 An alternative embodiment of the invention is schematically and partially shown in FIG. 6.
• a spatial multiplexing of the laser beams 8, 10 and 12 is implemented before focusing them on the target P.
• the last two mirrors 14 (top of FIG. 4) are replaced, which are associated with the beams 10 and 12, by two pierced mirrors 80 and 82, aligned with the last mirror 14 (top of FIG. 4), which is associated with beam 8.
• the pierced mirror 80 lets part of the beam 8 pass towards the target and reflects part of the beam 10 towards the latter.
• a beam stop means 84 is provided to stop the rest of the beam 10 (not reflected towards the target).
• the pierced mirror 82 the bore of which is larger than that of the mirror 80, lets part of the beams 8 and 10 pass towards the target and reflects part of the beam 12 towards the latter.
• a beam stop means 86 is provided to stop the rest of the beam 12 (not reflected towards the target).
• an achromatic focusing doublet 88 is provided for focusing the beams from aligned mirrors 14, 80 and 82 on the target.
• FIG. 7 Another variant embodiment of the invention is schematically and partially shown in FIG. 7.
• a beam stop means 94 is provided to stop the rest of the beam 10 (not reflected towards the target).
• the pierced mirror 82 is also replaced by another with a sharp edge 92 intended to reflect, towards the target, part of the incident beam 12, by letting part of the beams 8 and 10 pass to its target, around its periphery.
• a beam stop means 96 is provided to stop the rest of the beam 12 (not reflected towards the target).
• the achromatic focusing doublets 20, 22, 24 and 88 are advantageously studied to minimize aberrations. But they can be replaced by curved mirrors.

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